Growing Degree Day Models for Plant Growth Regulators Applied to Creeping Bentgrass and Kentucky Bluegrass.

Growing Degree Day Models for Plant Growth Regulators Applied to Creeping Bentgrass and Kentucky Bluegrass Authors: Henke, Soldat, Kreuser Plant Growth Regulators (PGRs) are widely used in the turfgrass industry. PGR efficacy has been related to more metabolism in the plant. Recent studies have shown applying trinexapac-ethyl based on of growing degree days (GDD) can predict more precise re-application intervals than standard calendar applications. The objective of this study was to determine if GDD models could be developed for PGRs on creeping bentgrass and Kentucky bluegrass fairways and quantify the impact of application rate on growth suppression. Treatments consisted of six PGRs at two application rates. Applications were made at the beginning of May, June, July and August on previously non-treated turfgrass at sites in Nebraska and Wisconsin. Higher applications rate generally resulted in higher percent suppression and longer re-application intervals. PGRs applied to creeping bentgrass at high rates reduced growth by 53 to 87%, while low rates reduced growth from 36 to 70%. Growth regulation was less effective on Kentucky bluegrass where the high rates suppressed growth by 23 to 63% and low rates reduced growth by only 18 to 48%. These results suggest that PGRs are less effective on Kentucky bluegrass fairways compared to creeping bentgrass fairways.